In color reproduction, half-tone color separations are used as copy originals for the production of offset- or relief-printing plates. Before the printing plates are exposed, the color separations are proofed using color proofing methods to check that the tonal values of the colors meet expectations in all parts of the image. If this is not the case, the color separations must be corrected accordingly.
To make corrections, the color separation, partially protected by a varnish, if necessary, is treated, e.g. with Farmer's reducer, which reduces the size of the half-tone dots by oxidative decomposition (dot-etching). This process previously has been used almost exclusively but has several disadvantages and is therefore being increasingly supplemented recently by dry retouching (dry dot etching).
Dry dot etching is based on the principle that a copy is made of the color separation to be corrected, the size of whose half-tone dots has been changed in the copying process by overexposure and underillumination. In this manner both plus and minus corrections are possible. If such corrections are only to be carried out in partial areas of the color separations, the remaining areas are protected by suitable masks. Such masks can be cut from masking-type films for simple designs and corrections to individual colors. With more complex designs and corrections to more than one color, photomechanically produced masks are used.
Such masks are usually produced by the compensation method: depending on the color tone to be corrected, a combination of half-tone color separation negatives and/or positives is bound up together, corresponding to the mask combination diagram shown in FIG. 1. The colors to be corrected, i.e., that must appear transparent on the mask combination, are entered on the outer ring. The required mask combination is given by the section of the circle assigned to this color and the two neighboring sections of the circle (see Deutscher Drucker No. 16, p. 47, 1984).
At most, therefore, three films of each color set (positives or negatives) are used, with each color only being represented once. After these films have been bound in accurate register, the combination, with diffusing sheets, e.g., Cronaflex.RTM.UC-7 Tracing Film (drafting film), E. I. du Pont de Nemours and Company, Wilmington, Del. or a suitable matt drafting film or other type light diffusing film, intercalated between the color separations (positive or negative) and the unexposed film to suppress the half-tone dot structure, is copied onto a silver halide photographic film, e.g., positive film, and after development gives the desired mask. Additional diffusion sheets can be inserted between the exposure source and the color separations. The mask exhibits a high degree of masking with the exception of the areas to be corrected. Now the actual tonal correction takes place, in that the color separation to be corrected is copied alone using a first exposure and is copied with the mask onto a suitable line- or half-tone silver halide screen film, e.g., Cronar.RTM. BLD-4 Bright Light Duplicating Film, Cronar.RTM. BLE-4 Bright Light Etchable Contract Film, both manufactured by E. I. du Pont de Nemours and Company, Wilmington, Del., etc. usng a second exposure.
This method is well suited to the correction of tonal values of relatively pure colors. It fails, however, with blackened colors (tertiary colors, i.e.. non-pure colors which contain all three basic colors, such as brownish), with similar color tones adjacent to one another that must be separated from one another, as well as especially in cases in which only the range of the middle tones is to be corrected without simultaneously influencing the range of the highlight- and shadow tones.
It is therefore an object of the present invention to give a process for the production of photographic masks that avoids the aforementioned disadvantages.